International Concrete Abstracts Portal

The break-off test is a recently developed nondestructive test for concrete. Although many experimental investigations have been carried out on this test, no in-depth theoretical evaluation has been done. In this study, the behavior of the break-off test specimen is investigated, and the theoretical basis of the test is explored. Based on linear elastic fracture mechanics, a model to predict the strength-manometer reading relationship of the test is proposed and compared with experimental results with good correlation. It was found that the ACI recommendation on the modulus of rupture (MOR) may be very conservative for certain members. The MOR of a rectangular beam is different from that observed from a circular cross section, such as the break-off test specimen. New MOR values are suggested for small rectangular beams and members with circular cross sections.

Fracture surfaces of concrete and mortar are irregular, tortuous, and stochastic in nature. To describe irregular and rough surfaces, quantitative fractographic parameters such as profile and surface roughness, fractal dimension, Fourier spectral analysis, etc., are often used. A fractal description of fracture surfaces of concrete and mortar by utilizing a new nondestructive technique, introduced by the authors, will be presented in this paper. Compact tension-fractured concrete specimens with a compressive strength of 46.8 MPa and a maximum aggregate size of 37.5 mm and a projected fracture area (ligament area) of 46,000 mm 2 (367.5 mm long by 125 mm wide), are analyzed. Through this technique, a microphotograph is taken and stored as a binary image using an image analyzer equipped with a stereo-microscope. The result is a topographical map of the fracture surface. Since the elevation of each point on the fracture surface is defined by its intensity value, the need for actual sectioning through the fracture surface, often employed, is eliminated. One-dimensional Fourier spectral analysis (1D FFT) to estimate the fractal dimension is carried out. To check the method of analysis, synthetic profiles with a known fractal dimension are generated. The results of the analysis suggest that concrete fracture surfaces are fractal for the range of scales considered, the digitized fracture surface images are found to mimic the actual fracture surfaces, their spectra follow a power lower behavior, and the technique is very promising and suitable for such materials.

Describes a large-scale pendulum device for testing reinforced concrete structures under impact loading. Details of the facility, including instrumentation of specimens and pendulum mass, are provided. Sample test results relative to full-scale bridge barriers and beams are presented. These tests show differences between responses under static and dynamic loads.

Describes tests that were performed to evaluate the feasibility of using the impact-echo method to evaluate the in-place strength of concrete in plate-like elements such as slabs and walls. In the impact-echo method, a stress pulse is introduced into an object by mechanical impact on its surface, and this pulse undergoes multiple reflections (echoes) between opposite faces of the object. The surface displacement of the object, caused by the reflected pulse, is monitored at a location adjacent to the point of impact, and the frequency of successive arrivals is determined. Knowing the thickness of the test object, the compression wave (P-wave) velocity is determined. A previously established concrete strength-P-wave velocity relationship can be used to estimate in-place strength. Results indicate that the impact-echo method can be used to determine P-wave velocity through a large volume of early-age concrete such as the slab specimens tested in this study. Use of the impact-echo method to nondestructively estimate the in-place strength of concrete is more appropriately limited to the estimation of early-age strength.

A new type of short-span bridge system has been developed and implemented over the Albermarle Sound south of Edenton, North Carolina. The new system incorporates precast flat-slab sections that are post-tensioned for continuity. The new system has the potential to replace traditional trestle-type bridges constructed using simple-span prestressed beams with a cast-in-place deck. A continuous two-span, half-scale model of the bridge system was built and tested under various load conditions. The bridge was evaluated analytically and experimentally for the transfer load case (dead load plus prestress), the maximum negative moment service load case, cracking load, and ultimate load. The model bridge performed as expected for all cases. Comparisons between analytical and physical models showed good correlation for all types of tests. At service load levels, the bridge exhibited a linear elastic response with no evidence of cracking. The ultimate load and deflections of the new bridge system were readily predicted by standard behavioral models for prestressed concrete.